1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display device having a high performance characteristic and a method of manufacturing the same.
2. Description of the Related Art
There has been developed a so-called In-Plane Switching (IPS) method in which an electric field parallel to a substrate is applied to a liquid crystal for an active matrix type liquid crystal display device. An IPS type liquid crystal display device has such advantages in that a wide angle of view can be obtained.
FIG. 14 shows one example of a plan layout of a unit pixel area included in an active matrix type liquid crystal display device according to the IPS method. FIG. 15 shows a cross section of the liquid crystal display device shown in FIG. 14 as sectioned along a direction PP. As shown in FIG. 15, the liquid crystal display device comprises a TFT substrate 100, an opposing substrate 200, and a liquid crystal 300. The liquid crystal display device is structured by filling a space between the TFT substrate 100 and the opposing substrate 200 which are set opposite from each other via a spacer and sealing member (both not illustrated) with the liquid crystal 300.
The TFT substrate 100 comprises a first transparent substrate 101 made of transparent glass or the like. Scanning lines 102 (not illustrated in FIG. 15) and common wirings 103 are formed on one surface of the first transparent substrate 101. As shown in FIG. 14, adjacent two scanning lines 102 having a predetermined space therebetween extend toward an X direction almost in parallel, and determine the X direction of the unit pixel area. The common wirings 103 extend almost in parallel with the scanning line 102, and are so arranged that two of the common wirings 103 sandwich one scanning line 102. That is, a unit pixel area has two common wirings 103 crossing thereinside. The two common wirings 103 are connected to each other by three common electrodes 111 which extend in the unit pixel area toward a Y direction almost perpendicularly to the common wirings 103. The common electrodes 111 include a center portion 111a which extends almost in the center of the pixel area, and edge portions 111b which extend in the both sides of the center portion 11a and have a larger width than that of the center portion 111a. 
In FIG. 15, there is shown an interlayer insulation film 104a which is formed on the first transparent substrate 101, the scanning lines 102, and the common wirings 103. Data lines 106 and a pixel electrode 112 are formed on the interlayer insulation film 104a. A semiconductor island 105 shown in FIG. 14 is also formed on the interlayer insulation film 104a. The semiconductor island 105 constitutes a TFT (Thin Film Transistor). The semiconductor island 105 is provided on the scanning line 102 via the interlayer insulation film 104a. 
The data lines 106 extend toward the Y direction almost perpendicularly to the scanning line 102, and determine the Y direction of the unit pixel area. The pixel electrode 112 is arranged in the center of the unit pixel area. The pixel electrode 112 includes two opposing portions 112a which extend toward the Y direction along the common electrodes 111, and two supporting portions 112b each of which is arranged so as to overlap a common wiring 103 and to support one edge of the opposing portions 112a. The opposing portions 112a of the pixel electrode 112 are arranged between the adjacent common electrodes 111 so as to oppose those common electrodes 111. Needless to say, as shown in FIG. 15, the interlayer insulation film 104a exists between the common electrodes 111 and the pixel electrode 112. Storage capacitors are formed between the common wirings 103 and the supporting portions 112b of the pixel electrode 112 which oppose each other via the interlayer insulation film 104a. 
A passivation film 104b is formed on the interlayer insulation film 104a, the data lines 106, the pixel electrode 112, and the TFT. An orientation film 116 which has been subjected to a surface alignment treatment is formed on the passivation film 104b. A polarizing plate 119 is provided on the other surface of the first transparent substrate 101.
The opposing substrate 200 includes a second transparent substrate 201. A black matrix 202 having an opening is formed on one surface of the second transparent substrate 201. The black matrix 202 is made of a material having a light shielding effect, and provided so as to oppose the data lines 106 which determine the unit pixel area. The opening of the black matrix 202 is covered by a color layer 203. A flattening film 204 and an orientation film 205 are formed on the black matrix 202 and the color layer 203. A conductive layer 207 and a polarizing plate 208 are formed on the external surface of the second transparent substrate 201.
This liquid crystal display device operates as follows. In order to drive the liquid crystal display device, a driver circuit (not illustrated) applies a gate pulse to scanning lines 102 sequentially, and applies a data signal whose voltage corresponds to the display tone to the data lines 106 almost synchronously with the gate pulse. A TFT which is connected to a scanning line 102 to which a gate pulse is applied (selected) scanning line 102 is turned on, and a voltage which is applied to the data lines 106 at this time is applied to the pixel electrode 112 via a drain electrode 107, the semiconductor island 105, and a source electrode 108.
When the gate pulse is cut off, the TFT is turned off. The voltage applied to the pixel electrode 112 at that time is stored in the capacitors between pixel electrode 112 and the common electrode 111, and between the common wirings 103 and the pixel electrodes 112.
Thus, the voltage which corresponds to the display tone is applied to the liquid crystal of each unit pixel area until the next selection period. While this voltage is applied, an electric field parallel to the substrate is formed between the common electrodes 111 and the opposing portions 112a of the pixel electrode 112, and the liquid crystal is oriented in a desired state. Therefore, the color of the color layer 203 is displayed in a desired tone.
As described above, in this liquid crystal display device, an electric field is formed between the common electrodes 111 and the opposing portions 112a of the pixel electrode 112, and this electric field parallel to the substrate is applied to the liquid crystal 300. However, the data lines 106 are also formed closed to, and along the opposing portions 112a of the pixel electrode 112. Thus, an electric field is formed also between the data lines 106 and the pixel electrode 112 due to the potential difference between them. Part of this electric field “leaks” to some parts of the liquid crystal 300 that are close to the data lines 106. The so-called leak electric field disturbs the orientation of the liquid crystal 300 and causes disclination, thus display quality is deteriorated.
It is undesirable that the electric field caused by the data lines 106 leaks to the liquid crystal 300. Thus, the wider edge portions 111b of the common electrodes 111 are provided to reduce this leak electric field. As shown in FIG. 15, the electric field caused by the data lines 106 is terminated mainly by the edge portions 111b of the common electrodes 111, not by the pixel electrode 112. Therefore, electric field leakage to the liquid crystal 300 is prevented.
However, in order to obtain a sufficiently high prevention effect (shield effect) against leakage, it is necessary to widen the width of the edge portions 111b of the common electrodes 111. The common electrodes 111 are usually made of a metal having a light blocking effect such as chromium or the like. Therefore, as the width of the edge portions 1111b is widened, a ratio of the display area to the unit pixel area of the liquid crystal display device, i.e., the aperture ratio is reduced.
A structure wherein a common electrode is formed above a data line, such as disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H11-119237, is proposed as a structure which can obtain a high shield effect while preventing reduction in the aperture ratio. FIG. 16 shows an example of a plan layout of a liquid crystal display device having such a structure. FIG. 17 shows a cross section of the display device shown in FIG. 16 when it is sectioned along a direction QQ. Components identical to those shown in FIGS. 14 and 15 are given the same reference numerals, and explanation for those components is omitted.
Unlike the liquid crystal display device shown in FIG. 15, in this liquid crystal display device, a pixel electrode 112 and a common electrode 111 are formed in a same plane above data lines 106.
As shown in FIG. 17, parts of the common electrode 111 are formed on a second interlayer insulation film 110 just above the data lines 106. As shown in FIG. 16, the common electrode 111 includes a supporting portion which overlaps a common wiring 103 shown in the upper side of FIG. 16 and extends toward an X direction, and two opposing portions which extend from the supporting portion toward a Y direction. The opposing portions have a length which is almost the same as a distance between two adjacent common wirings 103 existing in a unit pixel area. The common electrode 111 is electrically connected to the common wiring 103 via a contact hole 113 which penetrates a first interlayer insulation film 104 and the second interlayer insulation film 110.
As shown in FIG. 17, the pixel electrode 112 includes a first pixel electrode 109 formed on the first interlayer insulation film 104, and a second pixel electrode 112a formed on the second interlayer insulation film 110.
As shown in FIG. 16, the first pixel electrode 109 is formed in an H letter shape. That is, the first pixel electrode 109 has two linear portions arranged so as to overlap the common wirings 103, and a linear portion arranged so as to oppose the second pixel electrode 112 and to connect the two linear portions. A part of the first pixel electrode 109 is connected to a source electrode 108. A compensating capacitor is formed between the common wiring 103 and the first pixel electrode 109 which opposes the common wiring 103.
The second pixel electrode 112 includes three opposing portions, and a supporting portion for supporting the three opposing portions, and thus forms an E letter shape. The second pixel electrode 112 is arranged so as to engage with the common electrode 111 which is formed on the same surface. Adjacent two opposing portions of the second pixel electrode 112 sandwich one opposing portion of the common electrode 111a. The supporting portion of the common electrode 111 is arranged so as to overlap the common wiring 103 shown in the upper side of the FIG. 16, and is electrically connected to the common wiring 103 via the contact hole 113 for common electrode. The common electrode 111 and the second pixel electrode 112 are made of, for example, a material having an optical transmittance characteristic, such as ITO (Indium Tin Oxide) or the like.
In this liquid crystal display device, edge portions 111b included in the common electrode 111 that have a width wider than that of the data lines 106 are provided above the data lines 106. An electric field formed from the data lines 106 is terminated by the edge portions 111b of the common electrode 111 as indicated in FIG. 17 by arrows. Therefore, leakage of the electric field to the liquid crystal 300 is prevented. Thus, influence given on the electric field between the common electrode 111 and the pixel electrode 112 is reduced, and the deterioration of the displayed image is lowered.
However, since the data lines 106 and the edge portions 111b of the common electrode 111 oppose each other by almost the entire surfaces thereof via the second interlayer insulation film 110, electrostatic capacitance between the data lines 106 and the edge portions 111b of the common electrode 111 is relatively large. Thus, delay of the signal applied to the data lines 106 cannot be ignored.
To reduce such electrostatic capacitance, the second interlayer insulation film 110 between the data lines 106 and the edge portions 111b may be formed thicker. However, in this case, a longer time is required for forming the second interlayer insulation film 110, and thus the manufacturing throughput is lowered. And since the second interlayer insulation film 110 is formed thicker, a contact hole having a high aspect ratio will be formed. Thus, the yield is lowered, and the manufacturing cost is increased.
And since the opposing areas of the data lines 106 and the edge portions 111b of the common electrode 111 via the second interlayer insulation film 110 are large, there is a high possibility that an electrical short circuit (interlayer short circuit) is caused between the data lines 106 and the edge portions 111b due to a defect such as a pinhole caused in the second interlayer insulation film 110. The electrical short circuit increases the possibility that a line defect is caused when the display operation is performed.
In the above-indicated publication, an embodiment in which the edge portions of the common electrode are formed so as to cover a part of the data lines is also disclosed. However, in such a case where the edge portions of the common electrode are formed so as to overlap a part of the data lines, an electric field leaks to other parts of the unit pixel area than the data lines.
Moreover, in a case where the second interlayer insulation film 110 is made of an inorganic film such as a silicon oxide film or the like, the second interlayer insulation film 110 needs to be formed relatively thick, approximately 1 to 10 μm, since the dielectric constant is high. This brings about the same problems as described above. On the other hand, in a case where the second interlayer insulation film 110 is made of an organic film such as acrylic resin or the like, it can be formed to have a thickness of approximately 0.5 to 5 μm, since the dielectric constant is low. Thus, the problems caused by a thick film can be avoided. However, an organic film has a high permeability against ions. Thus, to prevent adhesion of ions to the back channel of a TFT, there is a limitation on materials which can be used as the organic film. And in a case where the second interlayer insulation film 110 is formed of a double-layered film made of an inorganic film and an organic film, there is a need to form openings respectively in the inorganic film and the organic film. Thus, manufacturing steps and manufacturing costs are largely increased.
To sum up, there has not conventionally been provided an active matrix type liquid crystal display device which can be manufactured without largely increasing the manufacturing steps and manufacturing costs, and which has lowered delay of a signal, and has decreased display defects.